Air impingement cooling has been used to manage the casing temperature of small gas turbines and to reduce and maintain the clearances between rotating blades and accompanying interior casing surfaces. One problem for air impingement cooling systems on heavy-duty gas turbines is the ability to achieve a uniform heat transfer coefficient across large non-uniform non-standard casing surfaces. On small gas turbines, small impingement holes and short nozzle to surface distances are normally applied. These factors produce the required higher heat transfer coefficients on the casing. One detrimental impact of applying small of impingement cooling holes is the need for operating with high differential pressure drop across the holes. This results in the requirement for undesirable high cooling air supply pressures which negatively impacts net efficiency.
Impingement cooling has been applied to aircraft engines as a method of turbine clearance control. However, the impingement systems used on aircraft engines cannot be used in heavy-duty turbine applications. The systems applied to aircraft engines utilize air extracted from the compressor as the cooling medium. It is not feasible to use compressor extraction air on heavy-duty gas turbines because the design heat transfer coefficients require cooler air temperatures. Heavy-duty gas turbines have a significantly larger, non-uniform casing surface that requires an intricate manifold design as compared to aircraft engines. Also, the casing thickness and casing thickness variations are considerably greater on heavy-duty gas turbines.
An impingement cooling system has been recently developed that can provide clearance control on heavy-duty gas turbines. This system properly delivers the required heat transfer coefficient to the targeted casing surface, and properly controls the flow of air to the casing.